1. Field of the Invention
The present invention relates to an actively controlled automotive suspension system which is capable of actively varying suspension characteristics and specifically to technologies for actively controlling lateral load shift to be shifted between left and right wheels and for actively controlling a distribution ratio of anti-rolling moment created at both front and rear suspension systems, depending on a road surface condition, such as snow, icy or wet roads and dry pavement, and a lateral acceleration exerted on the vehicular body during turns, so as to improve steering characteristics and to enhance cornering and driving stability of the vehicle.
2. Description of the Background Disclosure
Recently, there have been proposed and developed various active suspension control systems for automotive vehicles, which act in an active manner for suppressing vehicular attitude change, such as rolling, pitching, bouncing or the like, and thus achieve both riding comfort and driving stability. One such active suspension control system has been disclosed in Japanese Patent First Publication No. Heisei 2-3511. As is well known, such an active suspension system operates to generate anti-rolling moments at both front and rear suspension systems so as to suppress rolling of the vehicle on turns. The magnitude of anti-rolling moment created at front and rear suspension systems is varied substantially in proportion to the magnitude of a lateral acceleration exerted on the vehicular body. The conventional active suspension control system is utilized for a rear-wheel drive vehicle and comprises a controller for controlling a proportional gain for anti-rolling moment generated at a rear suspension system depending on a road surface condition, such as icy or wet roads and dry pavement such that a distribution ratio of anti-rolling moment of the rear suspension system to the front suspension system is reduced when the controller determines that the vehicle is travelling on a low road surface. That is, a lateral load shift provided at the rear suspension system is relative to that at the front suspension system during turns on a low frictional road surface so that cornering force created at the rear wheels is increased, i.e., the rear-wheel tires grip sufficiently on a low frictional road. As a result, both cornering stability and driving stability of the vehicle are enhanced during turns on a low frictional road.
In the previously noted conventional anti-roll controlling system, the road surface condition is detected by either a lateral acceleration sensor for monitoring a lateral acceleration exerted on the vehicular body or at least two wheel speed sensors for monitoring both a driven wheel rotational speed and a non-driven wheel rotational speed. A controller employed in the prior art anti-rolling controlling system would determine that the road surface condition corresponds to a low frictional road when the monitored lateral acceleration is within a designated small lateral acceleration range or when the rotational speed difference between front and rear wheel speeds exceeds a preset threshold. When the low frictional road condition is satisfied, the controller controls anti-rolling moments between both front and rear wheels in such a manner as to reduce the distribution ratio of anti-rolling moment of the rear suspension system to the front suspension system. Such an anti-roll control results in understeer tendencies of the vehicle during turns on a low frictional road. As is generally known, when the rear-wheel drive vehicle turns on a low frictional road in its critical operating state, the rear wheels (driven wheels) skid with a great speed difference between front and rear wheel speeds and therefore the rear wheels could throw the entire car into a rear-end skid. This could result in oversteer and/or spinning on wet or icy roads. As previously described, the conventional anti-roll controlling system could satisfactorily eliminate oversteer tendencies of the vehicle during turns on a low frictional road because of a smaller lateral load shift at the rear suspension system than that at the front suspension system. However, if a lateral acceleration sensor is utilized for judging a low frictional road surface condition, there is a possibility of malfunction of the controller employed in the anti-roll controlling system, during a moderate turn of the vehicle with a relatively small lateral acceleration. Due to malfunction of the controller, the distribution ratio of anti-rolling moment of the rear suspension system to the front suspension system is reduced, with the result that the vehicle may experience a stronger understeer during a moderate turn. Under this condition, great steering effort may be required due to malfunction of the controller.
On the other hand, if the rotational speed difference between front and rear wheel speeds is utilized for judging a low frictional road surface condition, detection accuracy for a low friction road condition is enhanced since the low frictional road condition is satisfied only when the driven wheels actually skid and the wheel speed difference exceeds a preset threshold. It is undesirable to occur stronger understeer by activation of the anti-roll controlling system even during a high-speed turn on a high friction road, such as dry pavement, i.e., even when the rear-wheels slip on a high friction road, due to an excessively great driving torque applied to the driven wheels. If the threshold of the wheel speed difference is set at a higher level, the number of activation of the anti-roll controlling system is extremely reduced, since the anti-roll control is achieved only when the controller detects an excessively great wheel speed difference. This results in an unsufficient cornering stability of the vehicle during turns on a low frictional road surface.
Therefore, it is desirable to provide an anti-roll controlling system having a high reliability of an anti-roll control, irrespective of a road surface condition, a lateral acceleration exerted on a vehicular body, and the wheel speed difference between driven and non-driven wheels.